1. Field of the Invention
This invention relates to a transmission mechanism for optical scanner and particularly to a scanner transmission mechanism that uses a screw bar to drive screw nuts to perform scanning operation.
2. Description of the Prior Art
Optical scanner is a widely used computer peripheral device which uses a chassis to capture image and a transmission mechanism to move the chassis for scanning a document.
FIG. 1 illustrates a conventional scanner transmission mechanism which generally includes a screw bar 10, a nut 20 and a chassis 30 fastened to the nut 20. The screw bar 10 is an elongate bar with threads 101 formed on outside surface. One end of the screw bar 10 engages with a motor 103 which may drive the screw bar 10 to rotate when activated.
The nut 20 has a round internal opening with internal threads (not shown in the figure) formed inside and is engageable with the threads 101. When the motor 103 is activated and rotates, the screw bar 10 will be turned and moves the nut 20 linearly which in turn moves the chassis 30 reciprocally to perform scanning operation.
However due to machining limitation, there will be tolerance between the threads 101 and the internal threads 202 of the nut 20 (the gap between thread 101 and 202 shown in FIG. 2). This gap will cause impact and vibration between the screw 10 and nut 20 during scanning operation.
For instance, when driving force is large, the screw bar 10 moves ahead and causes the front surface of the threads 202 making contact with the front surface of the threads 101 and result in a thread gap formed in the rear portion (shown in FIG. 2A). When friction resistance is large, the contact surface will take place between the rear surface of the threads 101 and 202, and result in a gap forming at the front position (shown in FIG. 2B). During scanning operation, the nut 20 will be moved reciprocally along the screw bar 20. The existence of the gap between the threads will cause vibration, which have negative impact on scanning quality and will lower scanner durability. The turning of the screw bar 10 also tends to tilt the nut 20 skew angularly (as shown in FIG. 2C) which may further down grade scanning quality. All of this begs for improvement.
It is an object of this invention to provide a transmission mechanism for scanner that does not have reciprocal vibration between the screw bar and nut for achieving higher quality scanning.
It is another object of this invention to provide a transmission mechanism for scanner that is free of vibration between the screw bar and nut for enhancing durability of the scanner.
It is a further object of this invention to provide a transmission mechanism for scanner that may prevent the nut from tilting angularly for enhancing scanning quality.
The transmission mechanism according to this invention includes a screw bar, a first nut, a second nut, a retainer, a first spring and a second spring. The screw bar is an elongate bar with threads formed on outside surface and has one end engaged with and driven by a motor for rotation. The first and second nut have internal threads engageable with the threads of the screw bar. The first nut is located between the motor and second nut.
The retainer is a hollow casing for housing the first and second nut inside. It has a front wall making contact with the first nut. The first spring is located and compressed between the first and second nut. The second spring is located and compressed between the second nut and a rear wall of the retainer. The second spring has greater spring force than the first spring. Hence the first nut is pressed with the rear surface of the internal threads making close contact with the threads of the screw bar. By the same token, the front surface of the internal threads of the second nut is making close contact with the threads of the screw bar. Therefore even when there is variation in driving or friction force in the transmission mechanism, the relative position between the first and second nut remains constant in the retainer. Vibration between the nuts and the screw bar thus may be avoided. Better scanning quality may be obtained and scanner durability may be improved. Furthermore, the retainer helps to prevent the nuts from tilting angularly which may further improve scanning quality.